Implantable medical devices typically have a metal case and a connector block mounted to the metal case which includes receptacles for leads which may be used for electrical stimulation or which may be used for sensing of physiological signals. Hermetically sealed within the case are the battery and the circuitry associated with the particular medical device, e.g., pacemaker circuitry, defibrillator circuitry, etc. Electrical feedthroughs are employed to connect the leads outside the metal case with the medical device circuitry and the battery inside the metal case.
Electrical feedthroughs serve the purpose of providing an electrical circuit path extending from the interior of a hermetically sealed metal case to an external point outside the case while maintaining the hermetic seal of the case. A conductive path is provided through the feedthrough by a conductive pin which is electrically insulated from the case itself. Such feedthroughs typically include a ferrule which permits attachment of the feedthrough to the case, the conductive pin, and a hermetic glass or ceramic seal which supports the pin within the ferrule and isolates the pin from the metal case. For example, illustrative feedthroughs are shown in U.S. Pat. No. 4,678,868 issued to Kraska, et al. and entitled "Hermetic electrical feedthrough assembly," in which an alumina insulator provides hermetic sealing and electrical isolation of a niobium conductor pin from a metal case. Further, for example, a filtered feedthrough assembly for implantable medical devices is also shown in U.S. Pat. No. 5,735,884 issued to Thompson, et al. and entitled "Filtered Feedthrough Assembly For Implantable Medical Device," in which protection from electrical interference is provided using capacitors and zener diodes incorporated into a feedthrough assembly.
Implantable medical devices can, under some circumstances, be susceptible to electrical interference such that functioning of the medical device is impaired. For example, medical devices may require protection against electrical interference from electromagnetic interference (EMI), electrocautery pulses, defibrillation pulses, electrostatic discharge, or other generally large voltages or currents generated by other devices external to the medical device. Such electrical interference can damage the circuitry of such medical devices or cause interference in the proper operation or function of the medical device. For example, damage may occur due to high voltages or excessive currents introduced into the medical device circuitry, e.g., pacemaker circuitry. Therefore, it is required that such voltages and currents be limited at the input of such medical devices, e.g., at the feedthrough. Protection from such voltages and currents is typically provided at the input of a medical device by the use of one or more zener diodes and one or more filter capacitors. For example, one or more zener diodes may be connected between the circuitry to be protected, e.g., pacemaker circuitry, and the metal case of the medical device in a manner which grounds voltage surges and current surges through the diode(s).
Such zener diodes and capacitors used for such applications may be in the form of discrete components mounted relative to circuitry at the input of a connector block where various leads are connected to the implantable medical device, e.g., at the feedthroughs for such leads. However, such protection provided by zener diodes and capacitors placed at the input of the medical device increases the congestion of the medical device circuits, particularly due to the need to provide a relatively high number of such discrete components, e.g., at least one zener diode and one capacitor per input/output connection or feedthrough, of the medical device. This is contrary to the desire for increased miniaturization of implantable medical devices, particularly in complex devices such as multiple chamber pacemakers, defibrillators, etc.
Further, generally, when such protection is provided, interconnect wire length for connecting such protection circuitry and pins of the feedthroughs to the medical device circuitry which performs desired functions for the medical device tends to be undesirably long. Such excessive wire length may lead to signal loss and undesirable inductive effects. Further, the wire length can act as an antenna that radiates undesirable electrical interference signals to sensitive circuits within the medical device to be protected. In other words, the wire length can reradiate interfering signals due to high current flow through the interconnect wires. This causes voltages to be induced in the high impedance CMOS circuitry of the implanted medical device. For example, such induced voltages can cause false sensing (e.g., under and/or oversensing) or can upset circuit function (e.g., cause a power on reset for a circuit).
Table 1 below lists some U.S. patents that use zener diode protection techniques:
TABLE 1 ______________________________________ U.S. Pat. No. Inventor(s) Issue Date ______________________________________ 5,735,884 Thompson et al. 7 April 1998 5,649,965 Pons et al. 22 July 1997 4,750,495 Moore et al. 14 June 1988 ______________________________________
All patents listed in Table 1, and elsewhere herein, are incorporated by reference in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Embodiments, and claims set forth below, many of the devices and methods disclosed in the patents of Table 1 and elsewhere herein may be modified advantageously by using the teachings of the present invention. However, the listing of any such patents in Table 1 is by no means an indication that such patents are prior art to the present invention.